1. Field of the Invention
The present invention relates to an optical power monitor used mainly in the field of optical communication.
2. Description of the Related Art
In recent years, remarkable technological innovation on information communication has been brought about, and a transition from communication by an electrical signal to communication by an optical signal is being made to meet the demand for increasing the communication speed and to cope with the increase in the amount of information with the proliferation of the internet. On many of trunk cables, information from a multiplicity of relay points concentrates. Therefore, the conventional trunk cables are being replaced with optical cables advantageous in terms of processing capacity and rate. Restudy of communication between optical cables and user terminals has been started and there is an increasing demand for implementing a more comfortable information environment at a reduced cost.
With the implementation of optical networks, high-speed information exchange has been enabled and new uses of optical networks have been expanded. Correspondingly, the amount of information transmitted through optical networks is being increased. To increase the amount of information processible with an optical fiber, a technique using a high-frequency signal for increasing the amount of signal per unit time or a technique called a wavelength multiplex system, i.e., a technique for simultaneously transmitting signals with a multiplicity of wavelengths for different information items through one optical fiber, is being used. Forming a dense and highly reliable communication network requires ensuring connections in many directions to a multiplicity of paths and using a plurality of optical fibers. Use of a plurality of optical fibers is indispensable from the viewpoint of maintenance.
For the formation of an optical communication circuit for transmitting a multiplicity of signals through an optical fiber, a wavelength division multiplex (hereinafter abbreviated as WDM) system is required in which a wavelength multiplexed optical signal is divided into signals of different wavelengths; optical signals of different wavelengths are multiplexed; and tapping and insertion of optical signals are performed. With the increase in amount of information, the importance of handled information is increased. With respect to a dropout in an optical signal, it is necessary to immediately identify the optical signal and the place where the dropout has occurred. It is also necessary to check the signal intensity as well as the existence/nonexistence of the connection for the optical signal. If the transmission distance is increased, the optical signal intensity decreases during transmission through optical fibers only. Therefore there is a need for an erbium doped fiber amplifier (hereinafter abbreviated as EDFA) for amplifying optical signals. The intensity of an optical signal externally supplied and the intensity of the optical signal issued to the outside after being amplified are measured with the EDFA to determine the amplification ratio. The provision of a monitoring function in each of light transmission circuit portions has become indispensable for the construction of a highly reliable optical communication system.
For monitoring of an optical signal, a method is used in which a portion of the optical signal is tapped by an optical coupler and the optical signal taken out by being tapped is detected by a photo-diode connected to the optical fiber. This method requires fusion splicing connection of each component, which is a hindrance to the reduction in the number of mounting steps. The optical coupler has a structure in which cores which are optical signal transmission portions of optical fibers are placed close to each other to enable tapping of an optical signal The length of the core portions close to each other is an important parameter of the amount of tapping. It is, therefore, difficult to reduce the size of the product. In particular, in recent designs, the number of wavelengths to be multiplexed is increased to increase the amount of information transmittable at a time. Since signal detection is performed after demultiplexing into wavelengths, the number of optical power monitors necessary for one unit is increased. Since the housing space in a unit assignable to optical power monitors is limited, a reduction in size of each power monitor is necessarily required.
For example, U.S. Pat. No. 6,603,906 discloses an optical power monitor of a reduced size. FIG. 17B shows the structure of the disclosed optical power monitor. FIG. 17A shows an example of an optical power monitor assembly 71 having a plurality of optical power monitors 70 mounted in a case 69. With an upper lid of the case removed. FIG. 17B is a longitudinal sectional view of the optical power monitor 70. Referring to FIGS. 17A and 17B, a multi-capillary glass ferrule 53 having two optical fibers 51 and 52 and a gradient index (GRIN) lens 54 are opposed to each other with a predetermined gap 55 formed therebetween. A filter 56 is formed on an end surface of the GRIN lens 54. The filter 56 reflects or transmits light passing through the GRIN lens 54. Light transmitted through the GRIN lens 54 passes through a gap 57 and is converted into an electrical signal by a photon detector or a photo-diode 58 to be taken out through terminals 59. Through an electrical output from the photon detector 58, the intensity of light in the optical path can be obtained. The multi-capillary glass ferrule 53 and the GRIN lens 54 are positioned by means of glass tubes 60 and 60′. The GRIN lens is a glass cylinder having its refractive index continuously and radially outwardly from its center axis. If light expands outwardly, the direction in which the light travels is bent toward the center axis.
The flow of light will be described with reference to FIG. 17B. Light entering the gap 55 from the optical fiber 51 (input light) passes through the GRIN lens 54 to reach the filter 56 on the GRIN lens end surface. Most of the light reaching the filter 56 is reflected, passes through the GRIN lens 54 and the gap 55 and enters the optical fiber 52 to become output light. The light transmitted through the filter 56 passes through the gap 57, enters the photon detector 58 and is converted into an electrical signal to be output through the terminals 59. This light path is indicated by the solid line arrow. Conversely, when light is input through the optical fiber 52, the light travels through a path similar to the above-described optical path and light (output light) is taken out from the optical fiber 51. This light path is indicated by the broken line arrow. The names referred to in the specification of the U.S. patent document have been used in this description. In the specification of the present invention, the photon detector is referred to as a photo-diode.
In the optical power monitor 70 shown in FIG. 17B, light is emitted (radiated) into the air at least once. Since the air has a refractive index different from that of the optical fibers, light radiated into the air is diffused. A lens typified by the GRIN lens is indispensable for collecting diffused light. Consequently, the product size of the optical power monitor depends on the sizes of the GRIN lens and the glass tubes. It is, therefore, difficult to reduce the overall size of the optical power monitor assembly 71 shown in FIG. 17A.
Japanese Patent Laid-Open No. 2003-329862 discloses an optical power monitor using a waveguide. FIG. 18A is a plan view of an optical waveguide module formed of the optical power monitors. FIG. 18B is a sectional view for explaining the principle of measurement of the energy of light with the optical power monitor. A plurality of waveguides 90 are formed in a substrate 81 generally parallel to each other. A channel 83 extending perpendicularly to the waveguides 90 is provided to divides the waveguides 90 into portion on the input side 86 and portions on the output side 87. A reflecting filter 84 is inserted in the channel 83 and a photo-detector 85 is disposed above the reflecting filter 84 on the input section 86. A planer waveguide type of optical circuit 80 is thus formed. Description will be made of measurement of the energy of light by using the flow of light with reference to the sectional view of FIG. 18B. The waveguide 90 has an upper cladding layer 91 and a lower cladding layer 93, with a core 92 interposed therebetween. Light traveling through the core 92 is emitted into the air in the channel 83. Most of light passes through the reflecting filter 84 to enter the core 92 on the output side 87. Part (indicated by the broken line) of the light is reflected by the reflecting filter 84 to enter the photo-detector 85. This light is converted into an electrical signal. The intensity of light in the optical path can be measured in this way.
It can be easily understood that in the planer waveguide type of optical circuit 80 disclosed in the above-mentioned Japanese patent document the thickness of the substrate holding the waveguides, the mechanism for holding the photo-detectors and so on is a hindrance to the reduction in size. It is also well known that the light loss in the portions connecting the waveguides and the optical fibers is large. It is difficult to reduce the loss. As a means for reducing the loss, replacement of the optical waveguides in the art disclosed in the Japanese patent document with optical fibers is easily conceivable. Even in a case where the optical waveguides are replaced with optical fibers, light entering from the input side is emitted into the air once, as in the planer waveguide type of optical circuit 80 disclosed in the Japanese patent document. Since the light emitted into the air is separated by the reflecting filter into light traveling to the output side and light entering the photo-detector, it is difficult to reduce the light loss.
Even with respect to the optical power monitor 70 disclosed in the above-mentioned U.S. patent document, which is considered to be a low-loss monitor, it is thought that the limit to the reduction in size of the single-channel optical power monitor 70 is 3.0 mm in diameter×20 mm as long as the assembly uses individual pigtail fibers, GRIN lens and photon detectors. As shown in FIG. 17A, the components are housed in the case 69 for multi-channel arrangement. The product size is further increased thereby and it is difficult to reduce the overall size.